Materials, such as thermoplastic resins, have been used to form pipes including, for example, those having a multilayer design. In some cases, the materials are heated, melted, or extruded, such as through the use of a die having a predetermined shape. One known example of a pipe extrusion die 100 is shown in FIG. 1. As illustrated, melted plastic may travel through a distributor portion 104 of the pipe extrusion die 100. From the distributor portion 104, the material may travel through a transition area 106 and then through a flair-out portion 108 of an extrusion nozzle 110. During this outward movement through the flair-out portion 108, the melted plastic material may flow through a flow passage 118, which may be defined by several die components, such as flow pins 112 and flow bushings 114. As the material flows outward toward the end 116 of the extrusion nozzle 110, the width of the flow passage 118 gradually decreases over its length.
As the material flows towards the transition area 106, such as between points A1 and B1, the flow properties of the material may remain constant. These flow properties which include, for instance, shear rate and shear stress, may be nearly identical. When the material flows toward point C1, however, the flow properties of the material may be much different than at point B1. This substantial change in material flow properties can cause variations in flow performance to occur as the material flows outward to a larger diameter, such as within the flair-out portion 108 between points B1 and C1. Flow variations also may occur as the material flows through a gradually decreasing passage, such as through flow passage 118.
The prior art extrusion die designs suffer from several deficiencies. For instance, conventional extrusion die designs fail to adequately control material flow properties, such as shear rate and shear stress. As a result, the end product is produced at higher temperatures and pressures, and has greater material thickness variation. Higher temperatures and pressures result in increased production costs. Greater material thickness variation necessitates an increase in raw material usage and, therefore, results in further increases in production costs. The inability to adequately control material flow properties in the prior art thus reduces the efficiency and increases the cost of the extrusion process.
Accordingly, it is desirable to provide an improved extrusion die apparatus and process for improving the efficiency and cost of the extrusion process.